1. Field of the Invention
The present invention generally relates to laser scanning systems for reading symbols such as bar code symbols and, more particularly, to a lightweight, multi-component, portable laser diode scanning head supportable by a user and aimable at each symbol to be read. Still more particularly, this invention relates to finding symbols and prompting the user when the symbols have been found.
2. Description of the Prior Art
Various optical readers and optical scanning systems have been developed heretofore to optically read bar code symbols applied to objects in order to identify the object by optically reading the symbol thereon. The bar code symbol itself is a coded pattern comprised of a series of bars of various widths, and spaced apart from one another to bound spaces of various widths, said bars and spaces having different light-reflecting characteristics. The readers and scanning systems electro-optically decoded the coded pattern to a multiple alpha-numerical digit representation descriptive of the object. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,409,470 and 4,460,120, all of which have been assigned to the same assignee as the instant application.
As disclosed in some of the above patents, a particularly advantageous embodiment of such a scanning system resided, inter alia, in emitting a laser light beam from a hand-held, portable laser scanning head which was supported by a user, aiming the head and, more particularly, the laser light beam, at a symbol to be read, repetitively scanning the laser beam in a series of scans across the symbol, detecting the scanned laser light which is reflected off the symbol, and decoding the detected reflected light. Inasmuch as the laser light beam was usually, but not always, generated by a helium-neon gas laser which emitted red laser light at a wavelength of about 6328 Angstrom units, the red laser light was visible to the user, and thus, the user, without difficulty, could properly aim the head and position and maintain the emitted red laser light on and across the symbol during the scanning.
However, in the event that the laser light beam was generated by a semiconductor laser diode, as, by way of example, see U.S. Pat. Nos. 4,387,297; 4,409,480 and 4,460,120, then the aiming of the head relative to the symbol was rendered more difficult when the laser diode emitted laser light which was not readily visible to the user. For some laser diodes, the laser light was emitted at a wavelength of about 7800 Angstrom units, which was very close to infrared light and was on the borderline of being visible. This laser diode light was visible to the user in a darkened room, but not in a lit environment where ambient light tended to mask out the laser diode light. Furthermore, if the laser diode light was moving, for example, by being swept across the symbol, and especially if the laser diode light was being swept at fast rates of speed on the order of a plurality of times per second, for example, at a rate of 40 scans per second, then the laser diode light was not visible to the user, even in a darkened room. Hence, due to one or more of such factors as the wavelength of the laser light, the intensity of the laser light, the intensity of the ambient light in the environment in which the laser light was operating, the scanning rate, as well as other factors, the laser diode light was rendered, in effect, "invisible", or, as alternately defined herein and in the claims, as being "non-readily visible".
This non-readily-visible laser diode light did not enable the user, however, to readily aim the laser diode light at the symbol, at least not without a great deal of difficulty and practiced effort because, simply put, the user could not see the laser diode light. The user, therefore, was required to hunt around by trial and error, hope that the scanning laser diode light was eventually properly positioned on and across the symbol, and wait until the scanning system advised him, typically by the lighting of an indicator lamp or by the sounding of an auditory beeper, that the symbol had indeed been successfully decoded and read. This hunting technique was a less-than-efficient and time-consuming procedure for reading symbols, particularly in those applications where a multitude of symbols had to be read every hour and every day.
Nevertheless, in the context of a laser scanning head which was desired to be made as lightweight, miniature, efficient, inexpensive sand easy to use as possible, the laser diode was more advantageous than the helium-neon gas laser, despite the non-readily-visible laser diode light characteristic, because the laser diodes were smaller, were lighter in weight, had reduced power requirements (voltage supplies on the order of 12v DC or less), were directly modulated for synchronous detection and for increased signal-to-noise ratios, etc., as compared to such gas lasers.
However, despite the above advantages, certain optical properties of the laser diode beam itself, aside from its invisibility, did not readily enable the laser diode beam to be focused to a desired spot size (e.g. a 6 to 12 mils circular spot) at a given reference plane exteriorly of the head, and to maintain said spot size within specified tolerances at either side of the reference plane within a predetermined depth of focus or field, i.e. the working distance in which a symbol located anywhere within the field can be successfully decoded and read. For example, the longer wavelength of the laser diode beam, as compared to that of the helium-neon gas laser, dictated a shorter working distance for the same spot size. The laser diode beam was also highly divergent, diverged differently in different planes, and was non-radially symmetrical. Thus, whereas the gas laser beam had the same small divergence angle of about one milliradian in all planes perpendicular to the longitudinal direction of beam propagation, the laser diode beam had a large divergence angle of about 200 milliradians in the plane parallel to the p-n junction plane of the diode, and a different larger divergence angle of about 600 milliradians in the plane perpendicular to the p-n junction. In the single transverse mode (TEM.sub.oo), the gas laser beam had a radially symmetrical, generally circular cross-section, whereas the laser diode beam has a non-radially-symmetrical, generally oval cross-section.
By way of example, in a so-called geometrical approach to solve the aforementioned focusing problem, and ignoring the non-radially symmetrical nature of the laser diode beam, optical magnification factors in excess of 80 were obtained if one wished to focus the beam spot to have about a 9.5 mil spot diameter at a reference plane located about 3-1/2" from the head. However, such high magnification factors dictated that, if one optical focusing element were employed (e.g. see U.S. Pat. No. 4,409,470), it would have to be critically manufactured, positioned and adjusted. If one employed several optical focusing elements in a lens system designed with a large numerical aperture, i.e. on the order of 0.25, as suggested by U.S. Pat. No. 4,387,297 to accept a large divergent laser diode beam and to distribute the magnification among the elements, then the mechanical tolerances for each element would be looser, and the positioning and adjustment procedures would be easier. However, a multiple, as opposed to a single, optical element design occupied more space within, and increased the weight and expense, of the head.
Also, although an oval laser diode beam spot was, in certain cases, desirable in ignoring voids in, and dust on, the symbol, as well as in rendering the light-dark transitions more abrupt, as compared to a circular gas laser beam spot during a scan across a symbol, these advantageous features occurred when the longer dimension of the oval spot was aligned along the height of the symbol. Thus, to obtain such desirable features, the laser diode beam had to be correctly aligned in a certain orientation relative to the symbol. In a situation where the symbols were oriented in a random manner relative to the laser diode beam, the head had to be frequently manipulated to correctly orient the laser diode beam on the symbol, and this further aggravated the already-less-than-efficient and time-consuming procedure for reading symbols, particularly on a mass basis, with laser diode light. Although it was possible to circularize the oval laser diode beam spot using an anamorphic collimator, this further increased the number of optical elements, the space, the weight and the expense.
Still another drawback inherent in earlier laser scanning heads, both of the gas laser and laser diode type, was that they were not readily adaptable to different applications. Different end users had different requirements. Whereas one user might want the electronic circuitry for decoding the detected reflected light to data descriptive of the symbol, and for controlling the decoding, to be mounted in the head, another user would require this electronic circuitry to be located remotely from the head. Still other users had different requirements concerning whether or not to locate a rechargeable power source or a data storage either locally in, or remote from, the head. Thus, the prior laser scanning systems had, more or less, to be custom-made for each user, and this was not altogether desirable in terms of manufacturing or marketing. Also, if the user wished to change the system requirements, the user had to forego the change, or obtain another system.
Yet another disadvantage associated with earlier laser scanning heads was that each had a discrete light-transmissive window mounted thereon. The discrete window was a separate piece which had to be glued in place and, hence, over time, the window sometimes became free of its glued mounting, particularly if the head was frequently subjected to mechanical shock and abuse. Once the window became disengaged, moisture, dust and other such contaminants were free to enter the interior of the head, thereby coating the optics and the electronic circuitry therein, and possibly interfering with their intended operation.